A wide gap semiconductor material such as silicon carbide (SiC) has various excellent characteristics such as dielectric breakdown strength about 10 times as high as that of silicon (Si), and is therefore drawing attention as a suitable material for a high breakdown voltage power semiconductor device having a high reverse breakdown voltage characteristic.
Power semiconductor devices are classified into: unipolar elements such as a Schottky diode, a MOSFET, and a JFET; and bipolar elements such as a pn diode, a bipolar transistor, an IGBT, and a GTO thyristor. For each of these elements, SiC can serve to achieve significant reduction of power loss as compared with the case of Si.
The breakdown voltage of an element of a power semiconductor device depends on not only the breakdown voltage of an active portion but also the breakdown voltage of an outer circumferential termination structure. In view of this, there is disclosed a SiC semiconductor element including a junction termination structure including: a breakdown voltage maintaining layer having a first conductivity type; and a region having a finite length and having a second conductivity type different from the first conductivity type, wherein the junction termination structure is formed such that an impurity concentration is spatially modulated and the impurity concentration tends to be decreased gradually (Patent Document 1, FIG. 3).
Moreover, there is disclosed a semiconductor device in which a termination structure constituted of a pn junction is provided in the outer circumference of a semiconductor element, wherein the concentration of an impurity having a second conductivity type is more reduced as it becomes deeper from a predetermined depth of the impurity region, and a degree of the reduction is more gradual at the end portion than that in the center portion (Patent Document 2, FIG. 1).